The latest installment...
Just received the birth certificate of Hélène - the name on the certificate is the Anglicised Helen Margaret by the way, but the really interesing part, as may be supposed is the mother's name:
Mary Broadwood formally HAMILTON

This, of course set me thinking -
In her daughter's marriage banns in France she's Marie Broadwood Tourmaly Hamilton
The image looks like Tormaly or Formaly: I was wondering if it was just a misunderstood 'formerly'
I think that's the case - if Hélène just gave her birth certificate when registering for her marriage in Paris then the person completing the banns may just have copied the name. Thanks for that Tati.

Then I got to thinking further, that as the certificates I have are from the copied register rather than the original, could there have been a transcription error when copying the original: there are some similarities in the letter formations of Hamilton & Lowther when written in script.
Where does this leave me now, and where do I go from here?
I am, of course, much further on thanks to the Rootschat community - less than a month ago I had no definite knowledge of the daughter's births or Frances Mary's marriage - you've helped me find them so huge thanks.

On all three certificates, Mary goes by her married name of Broadwood, but it's variously Hamilton, Lowther, or nothing entered.
My instinct is that Mary WAS the wife of Charles Broadwood, used his middle name of Hamilton to suit her and that Lowther, on Tatiana's birth certificate is a mis-transcription.
Any more thoughts to help me along?